It is known that the power output elements of an amplifier/transmitter can be built using a plurality of transistors, for example, with more than 100 transistors, which act connected in parallel. It is important that the load on the individual transistors of this circuit be as uniform as possible so that no single transistor be overloaded. Due to the unavoidable manufacturing tolerances of the available transistors (of the same type) and, in particular, due to their different gate threshold voltages, which may vary from 2 V to 4 V, it is essential that proper balancing be provided. Even differences as small as 20 mV in the gate bias between transistors result in differences of approximately 20% in the respective bias currents. Since it is not feasible to select, with sufficient accuracy, so many transistors having the same threshold voltages, one solution is to individually adjust the operating point of each transistor. In principle, either pre-adjusted control elements or electronic regulating circuits can be used for this purpose. Manual balancing of a number of potentiometers is, however, extremely costly. While electronic actuators with ROMs and digital-analog converters make automatic adjustment possible, ultimately they also require an extremely high expenditure for the individual DA converters and their drivers.
Another problem arising with a circuit consisting of a plurality of transistors is the temperature-dependence of the threshold voltage of the respective transistors. Since the temperature coefficient is mostly negative, i.e., the current increases with increasing temperature, catastrophic increase in the power loss may occur as a result of this positive feedback.
Therefore, when the transistors of a power amplifier element are connected in parallel, current may become concentrated in one or a few of the transistors even in the case of small parameter differences and/or temperature changes, and even if the total current of said element is limited by other measures. Aging effects of the transistors proper or of their bias sources may also result in considerable current variations, even if such aging effects cause only little change in the threshold voltage or the bias.
In operating an amplifier element, in particular, a power output element of an amplifier, in particular, a transmitter, the minimum object and requirement consists of a linear reproduction of the amplitudes of the high-frequency signals to be amplified. It is known that such an amplifier element may be operated in Class A or B operation. The selection of one of the aforementioned operating modes depends, in particular, on whether small-signal or large-signal modulation is performed and what bias current is required or tolerated for these amplifier elements.
In the case of small modulation amplitudes, Class A operation is preferably used for both half-waves of the high-frequency signal in a suitably selected, preferably linear range of the characteristic curves. The bias current set determines the initial steepness. If this is selected to be one-half of the average large-signal steepness of a transistor, approximately the same gain is obtained for small and large signals with a so-called A-B Class operation. The absolute value and constancy over time of the bias current are critical for low distortion and high stability of the output signal.
A special application of an amplifier with a power output element is in a pulse transmitter. Such pulse transmitters are used, for example, in magnetic resonance tomography for generating radio frequency magnetic fields. The output pulses of such a transmitter have a transmission time lasting for one or a few ms, for example, with pauses between the pulses, which are the periods for reception of the response signals. In order to prevent interfering output noise of the transmitter from appearing in the transmission pauses/reception periods, it is customary to shut off the bias current of the transmitting element, which is referred to as blanking. This also contributes to a highly desirable reduction in the average DC power consumption of the transmitter power output element. Switching between transmission and pause, referred to as gating, takes place via an external control signal. The goal is to achieve a switchover delay that is as short as possible, e.g., less than 10 .mu.s.